Many communication systems adopt a master-slave, command-response type of protocol whereby a master unit sends a command to one or more slave devices and solicits their responses. When there are more than one slave replies simultaneously, the slave signals arrive at the master unit concurrently so the master receives a corrupted signal and cannot decode the slaves' responses. Even in the slotted-Aloha system where the slaves can randomly choose a time slot out of a finite number of time slots to reply, the probability that two or more slaves choose the same time slot is not small. When signal collision occurs, the master has to poll the slaves again and that will reduce the system throughput.
The Radio Frequency Identification (RFID) system is one such system. The master is the RFID reader and the slaves are the RFID tags. In practice, there can be many tags within the receiving range of a RFID reader so the collision problem is a major issue. There have been many anti-collision schemes developed over the years. Many adopt signal diversification approach such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiple Access (OFDMA) and Spatial Division Multiple Access (SDMA). Yet most of these schemes require rather complex circuitry at the RFID tag to realize them. However, complex circuitry requires more power to operate. For those RFID systems whereby the tags are energized solely by the reader, this means that the read distance will be reduced. More importantly, more circuitry means higher cost to produce the tags. As the overall cost-benefit analysis of an RFID deployment depends heavily on the cost of the tag, all these approaches are not very attractive. Hence, a more cost effective collision avoidance or collision mitigation solution is needed.